(1) Field of the Invention
This invention generally relates to a velocity reduction method for reducing the flow-induced noise of towed sensor systems. More particularly, the invention relates to a method for reducing the tow velocity of a hydrophone system while keeping the tow vessel speed constant and therefore reducing the flow-induced noise received by the hydrophones without affecting ship trajectory or operation. By reducing the flow-induced noise, higher quality data can be obtained.
(2) Description of the Prior Art
Arrays of pressure sensors are used in both commercial and military systems for the reception of sound waves in water, air, or other fluids. The array is a multi-sensor system which allows for the simultaneous acquisition of signals from spatially separated locations. Commercial towed systems use hydrophone arrays to locate oil deposits beneath the ocean floor. Military systems use pressure sensor arrays to locate and classify acoustic targets (e.g., a ship or submarine in water or a tank in air). A fundamental limitation of towed systems of pressure sensors is the flow-induced noise. This non-acoustic noise is generated by pressure fluctuations at the solid/fluid interface of the structure containing the sensors. Some part of the noise is transmitted through the structure and received by the pressure sensors. The resulting unwanted noise is termed, "self-noise" because it is the resulting noise of the system in the absence of any acoustic signals. The flow-induced noise is directly related to the speed of the array system relative to the surrounding fluid. Turbulence scaling has shown that the magnitude of the direct flow noise is proportional to the velocity cubed (U.sub.0.sup.3) when a fully developed turbulent boundary layer is present.
By reducing or eliminating the self-noise induced by the flow of fluid relative to the pressure sensors, performance of the pressure sensors (e.g., detection range or signal-to-noise ratio) can be maximized. The following patents, for example, disclose various attempts at noise control in arrays:
U.S. Pat. No. 2,729,300 to Paslay et al; PA1 U.S. Pat. No. 3,281,767 to Cryar; PA1 U.S. Pat. No. 3,286,225 to Huckabay et al; PA1 U.S. Pat. No. 4,314,363 to Thigpen et al; PA1 U.S. Pat. No. 4,566,083 to Thigpen; PA1 U.S. Pat. No. 4,570,245 to Thigpen; PA1 U.S. Pat. No. 4,581,723 to Savit; PA1 U.S. Pat. No. 5,113,376 to Bjerkoy; a nd PA1 U.S. Pat. No. 5,345,522 to Vali et al.
Specifically, Paslay et al. disclose a water borne means for making seismic surveys of underwater deposits of oil by seismic signals received by a plurality of detecting devices disposed within an elongated flexible streamer. The streamer is slightly positively buoyant when immersed in the water and is provided with a plurality of weights suspended therefrom at intervals and adapted to cause the streamer assembly to be neutrally buoyant when towed through the water at a high rate of speed by a moving vessel whereby the streamer slowly sinks through the water at the points of connection with the weights as the forward end of the streamer is momentarily brought to rest during the towing operation and the sections of the streamer intermediate the weights slowly rise within the water. An arrangement is thus provided in which only the forward end of the streamer is momentarily brought to rest, the remainder of the streamer continuing forward movement as a shot is fired and the entire length of the streamer is maintained in tension during the reception of the seismic signals whereby fortuitous noises of spurious character caused by a release of the towing strain within the streamer and extraneous noise signals from the lead-in connecting the streamer and the vessel are prevented from being picked up by the detecting devices. Accordingly, the patent to Paslay et al. describes a system in which the array is designed to stop and physically touch the ocean floor. The weighted streamer is used to sink the array when its motion is minimized, thus the array is not maintained at a constant depth.
The patent to Cryar discloses a method and apparatus for continuous marine seismic surveying, in which a seismic impulse source is transported along the surface of a marine body and produces seismic impulses at intervals, the reflections of which from the marine bottom and sub-bottom strata are detected and recorded to produce a profile of the subsurface conditions. Accordingly, Cryar describes the simultaneous deployment and retrieval of two hydrophones or groups of hydrophones (an array) such that one is "always at a condition of minimum movement in the water." However, there is no consideration or understanding in Cryar of the effect that the negatively buoyant tow cable therein will have on the system, and two separate arrays must be used.
Huckabay et al. disclose a continuous marine seismic surveying system whose intent is to improve the signal-to-noise ratio in continuous marine seismic surveying by periodically stopping or slowing the motion of the towed detector member, so that during a portion of the survey operating cycle the hydrophone array can be dead in the water.
During this portion of the cycle, the requisite function of transducing the returned pressure waves can take place with little or no noise interference generated in the hydrophones.
The system has the capability of conducting a continuous survey of an area from a craft moving at a substantially constant speed. Although Huckabay et al. describe the basic concept of "stopping or slowing" the motion of the towed array to improve a signal-to-noise ratio, only the use of a neutrally buoyant array or "streamer" is described. Accordingly, Huckabay et al. fail to recognize or address the effect of a non-neutrally buoyant tow cable on the array shape.
Thigpen et al. disclose a marine seismic cable handler which is a hydraulically-powered yo-yo reel for handling a marine seismic cable in shallow water. At the beginning of a recording episode, a partially reeled-in seismic cable is released to drift to a standstill behind a ship. Data are recorded. The cable is then accelerated to match the ship's velocity. Following the initial acceleration, the cable is super-accelerated as it is partially reeled in. A microprocessor adjusts a dwell time between recording episodes to compensate for small variations in the ship's velocity. It is acknowledged that the patent to Thigpen et al. describes the basic concept of deploying the tow cable to decelerate the array to a standstill and mentions the need for maintaining a constant array depth; but proposes achieving these results via chains or buoys and does not describe a system using a neutrally buoyant tow cable.
Thigpen '083 discloses a seismic timing control system for optimizing the acoustic signal-to-noise ratios during a useful maximal-length recording cycle within the time frame of a minimal-length recording episode. To implement the method, during an intermittent-tow seismic operation, the noise level due to cable manipulation during an entire recording episode is monitored. A quiet-window of a desired length is established by adjusting the length of the interval during which the seismic cable is at rest. The initiation of a recording cycle is adjusted relative to the quiet window to take maximum advantage of the quiet period. Although Thigpen '083 describes the basic concept of deploying the tow cable to decelerate the array to a standstill and mentions the need for maintaining a constant array depth, these results are again obtained by provided chains or buoys to the array. Thigpen '083 does not describe a system using a neutrally buoyant tow cable.
Thigpen '245 discloses a constant tensioner for a seismic marine cable or towed seismic cable, wherein the instantaneous towing force and the instantaneous cable displacement relative to a fixed shipboard reference are continuously measured and sampled. A microprocessor associated with a servo controlled capstan device uses those measurements to pay out cable or to retrieve cable in response to an increase or decrease in the instantaneous towing force. The microprocessor also tries to minimize the average cable displacement. Accordingly, Thigpen '245 describes a system to maintain constant tow cable tension via deployment and retrieval of the tow cable. This system is only for small displacement and not the gross motions needed to effect flow-induced noise reduction.
The patent to Savit discloses a method for maintaining a substantially constant tension on a towed seismic cable, wherein the instantaneous towing force and the instantaneous cable displacement relative to a fixed shipboard reference are continuously measured and sampled. A microprocessor associated with a servo controlled capstan device uses those measurements to pay out cable or to retrieve cable in response to an increase or decrease in the instantaneous towing force. The microprocessor also tries to minimize the average cable displacement. As with Thigpen '245, Savit is primarily describing a system to maintain constant tow cable tension via deployment and retrieval of the tow cable and is thus only for small displacement.
The patent to Bjerkoy discloses a method for conducting seismic surveys in waters covered with ice. Seismic impulses are actuated under water and reflected signals are detected by a streamer cable towed behind a vessel in order to avoid the source of noise located outside of the seismic system due to the ice-breaking operation of the vessel. The vessel is stopped during active survey and the streamer cable is hauled in with a speed corresponding to the desired propulsion speed of the cable during detection. After detection, the vessel again resumes ordinary operational speed and the streamer is paid out with a speed which maintains the desired advancing speed of the system. Bjerkoy is therefore confined to the retrieval of an array when the tow vessel is not moving. This method cannot be used for a continuously moving vessel.
Vali et al. disclose a reduced noise fiber optic towed array and method of using the same wherein the fiber sensors are connected in parallel, and the optical fiber cable is paid out from the towing ship at a velocity about equal to but opposite to the velocity of the towing ship. Turbulence and acceleration noise is reduced due to the manner in which the fiber cable is paid out, and crosstalk between sensors is eliminated. Although Vali et al. describe the basic concept of paying out the tow cable to reduce the array velocity to zero, a negatively buoyant tow cable as used therein will cause the front end of the array to sink dramatically when the speed of the array is reduced to zero or near-zero.